1. Field of the Invention
This invention relates to a modulator, and more particularly a delta-sigma modulator.
2. Description of the Related Art
FIG. 1 illustrates a conventional delta-sigma modular proposed by K. Lee, M. R. Miller, G. C. Temes, in “An 8.1 mW, 82 dB Delta-Sigma ADC with 1.6 MHz BW and −98 dB THD,” Proc. IEEE Custom Integrated Circuits Conf. (CICC), pp. 93-96, September 2008. The conventional delta-sigma modulator converts an analog input signal (U(z)) to a digital output signal (V(z)) using delta-sigma modulation. The conventional delta-sigma modulator includes a loop filter 11, a noise coupler 12, a quantizer 13 and a digital-to-analog converter 14. The loop filter 11 is a second-order filter.
A relationship between the analog input signal (U(z)) and the digital output signal (V(z)) can be expressed as the following equation:V(z)=STF(z)·U(z)+NTF(z)·(1−z−1)·Q(z),where STF(z) is a signal transfer function attributed to the loop filter 11, NTF(z) is a noise transfer function attributed to the loop filter 11, 1−z−1 is a noise coupling function attributed to the noise coupler 12, and Q(z) is a quantization noise introduced by the quantizer 13. It can be appreciated from the aforesaid equation that the noise coupler 12 can increase the order of noise shaping of the conventional delta-sigma modulator by one. In other words, the conventional delta-sigma modulator can achieve third-order noise shaping.
Referring further to FIG. 2, the conventional delta-sigma modulator is implemented using switched-capacitor techniques. The analog input signal (U(z)) is a single-ended signal. The noise coupler 12 includes an amplifier, three capacitors (CH1, CH2, CF3) and twelve switches, which are operable based on eight control signals (φodd, φeven, φ1o, φ1e, φ2o, φ2e, φ1D, φ2). Therefore, due to the configuration of such noise coupler 12, the conventional delta-sigma modulator has a relatively complicated circuit structure, thereby increasing the complexity in circuit control.